High pressure diecasting (HPDC) is widely used to mass produce metal components that are required to have close dimensional tolerances and smooth surface finishes. One disadvantage, however, is that parts produced by conventional HPDC are relatively porous. Internal pores arise because of shrinkage porosity during solidification, and also the presence of entrapped gases such as air, hydrogen or vapours formed from the decomposition of die wall lubricants.
Castings made from HPDC aluminium alloys are not considered to be amenable to heat treatment. This follows because the internal pores containing gas or gas forming compounds expand during conventional solution treatment at high temperatures (eg. 500° C.) resulting in the formation of surface blisters on the castings. The presence of these blisters is visually unacceptable. Furthermore, expansion of internal pores during the high temperature solution treatment may have adverse effects both on the dimensional stability and mechanical properties of affected high pressure die castings.
As discussed in Altenpohl “Aluminium: Technology, Applications, and Environment”, Sixth Edition, published by The Aluminium Association and The Minerals, Metals and Materials Society, —see pages 96 to 98—there are techniques which allow high pressure die castings to become relatively pore-free and thus heat treatable in the absence of blistering. These techniques include vacuum die casting, pore-free die casting, squeeze casting and thixocasting, all of which involve cost penalties.
Of these techniques, vacuum systems are applied most frequently, with the aim of reducing porosity within the casting. In many cases the remaining level of porosity is still too high to allow heat treatment. However, there are some exceptions.
For example, in U.S. Pat. No. 6,773,666 to Lin et. al., an improved Al—Si—Mg—Mn alloy is disclosed as able to be high pressure die cast using Alcoa's AVDC die casting technique to produce extremely low porosity in resultant castings. The alloy composition contains less than 0.15Fe, less than 0.3Ti, less than 0.04Sr, and is substantially copper free, chromium free and beryllium free. It is similar to the casting alloy AA357 as well as the Australian casting alloy designations CA601 and CA603 (Aluminium Standards and Data—Ingots and Castings, 1997). The AVDC method uses very high vacuum pressure to produce components that are relatively pore free, and are reported as being weldable and heat treatable (see, for example, http://www.alcoa.com/locations/germany_soest/en/about/avdc.asp, 2005). In the prior art of Lin et. al., the castings were examined by X-ray analysis and found to be in excellent condition in regards to porosity contents. This high vacuum casting technique, followed by the heat treatment stages of solution treatment from 950-1020° F. (510-549° C.) for 10-45 minutes, quenching into water at 70 to 170° F. (ambient to 77° C.) and artificial ageing for 1-5 h at 320-360° F. (160-182° C.) was believed to achieve adequate properties for aerospace applications. Following the heat treatment schedules taught within this prior art, minor blistering was reported to have appeared upon the surfaces of the alloy examined, and was believed to have resulted from entrapped lubricant. However, the alloy was disclosed as being of high structural integrity and deemed suitable for aerospace applications.
Another example of a technique to reduce or remove porosity and thus facilitate heat treatment is disclosed in U.S. Pat. No. 4,104,089 to Miki wherein components produced from Al—Si—Mg—Mn alloy were able to be heat treated conventionally following a pore-free diecasting process. That diecasting process is based on earlier work evidently that of U.S. Pat. No. 3,382,910 to Radtke et al in which the die cavity is purged with a reactive gas that combines with the molten metal to reduce the level of porosity in resultant castings.
The conventional heat treatment procedure for aluminium alloys normally involves the following three stages:    (1) solution treatment at a relatively high temperature, below the melting point of the alloy, often for times exceeding 8 hours or more to dissolve its alloying (solute) elements and homogenise or modify the microstructure;    (2) rapid cooling, or quenching, such as into cold or hot water, to retain the solute elements in a supersaturated solid solution; and    (3) ageing the alloy by holding it for a period of time at one, sometimes at a second, temperature suitable for achieving hardening or strengthening through precipitation.
The strengthening resulting from ageing occurs because the solute taken into supersaturated solid solution forms precipitates which are finely dispersed throughout the grains and which increase the ability of the alloy to resist deformation by the process of slip. Maximum hardening or strengthening occurs when the ageing treatment leads to the formation of a critical dispersion of at least one type of these fine precipitates.
An alternative to the heat treatment procedure mentioned above is what is known as a T5 temper. In this case, the alloy is quenched immediately following casting while it retains some of its elevated temperature, and then artificially aged to produce more moderate improvements in properties.
Solution treatment conditions differ for different alloy systems. Typically, for casting alloys based around Al—Si—X, solution treatment is conducted at 525° C. to 540° C. for several hours to cause appropriate spheroidisation of the Si particles within the alloy and to achieve an appropriate saturated solid solution suitable for heat treatment. For example, Metals Handbook, 9th ed. vol. 15 p. 758-759 provides times and temperatures typical for solution treatment of casting alloys to provide these changes. Typically, the time of solution treatment for the alloys based on Al—Si—X is given as being between 4 and 12 hours, and for many alloys 8 hours or more, depending on the specific alloy and temperature of solution treatment. The time of solution treatment is normally considered to commence once an alloy has reached within a small margin of the desired solution treatment temperature (eg. within 10° C.), and this can vary with furnace characteristics and load size. However, this process, if applied to conventional aluminium alloy high pressure diecastings, is unsuitable because it will cause substantial unacceptable surface blistering on the diecastings.